JP3381840B2 - Viscosity determination method for density measurement - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for determining the viscosity when correcting the density of a test substance based on the viscosity in the density measurement of a vibration type densitometer.

[0002]

2. Description of the Related Art A U-shaped tube having fixed base ends (both upper ends of U-shape) is filled with a substance to be tested (liquid or gas) and mechanical vibration is applied to the U-shaped tube. When given, the vibration will occur at a frequency corresponding to the density of the test substance filled in the U-shaped tube.
Vibrate . Therefore, the vibration frequency (or vibration period)
Is measured, the density of the test substance can be obtained. The vibrating densitometer applies this principle.

FIG. 2 shows the basic structure of a conventional vibrating density meter. Tip of U-shaped tube 10 (lower part of U-shaped)
The permanent magnet 11 is fixed to the U-shaped tube 10 and the U-shaped tube 10 is vibrated by passing a current of a predetermined frequency through the drive coil 14 arranged near the permanent magnet. The vibration of the U-shaped tube 10 is detected by the sensor 13, and the detection signal from the sensor 13 is amplified by the amplifier 12 and returned to the drive coil 14. With this configuration, the U-shaped tube resonates at the frequency of the current flowing through the drive coil and vibrates.
Further, the vibration cycle of the U-shaped tube is measured based on the output signal from the sensor 13 and is used for the density calculation in the calculation means 15.

By the way, in the density obtained simply from the vibration frequency as described above, a density error depending on the viscosity of the test substance occurs as shown in FIG. That is, the larger the viscosity η, the larger the error rate indicated by Δρ / ρ ₁ in FIG. 4 (when the true value of the density is ρ ₀ and the measured value is ρ ₁ , Δρ =
ρ ₁ −ρ ₀ ).

In order to correct the density error depending on the viscosity η, it is necessary to obtain the viscosity of the test substance. Here, the viscosity η can be obtained from the relationship with the damping constant of the vibration. TRANSACTIONAL ON INDUSTRIAL ELECTRONICS
AND CONTROL INSTRUMENTATION, VOL.IEC1-27, NO.3, AUGU
ST 1980, 247-253 (reference 1). That is, the viscosity-damping constant characteristic is, for example, as shown in FIG.
As shown in, the functions are represented by b0 = f (η) and b1 = g (η). Here, the 0th order oscillation is a mode in which the base end portion of the U-shaped tube is a node of vibration as shown by i = 1 in FIG.
Usually, the density is obtained from the frequency in this mode. Further, the primary vibration is vibration having a node at the base end portion and a position about 1/4 from the base end to the base end as shown by i = 2 in FIG. In addition, higher-order vibration modes (i = 3, i =
There is also 4).

In order to form the vibrations of the above modes,
The above-mentioned Document 1 has a configuration driven by a circuit as shown in FIG.

That is, the piezoelectric element 21 as a sensor
Of the modulator 2 as a signal U via the variable gain amplifier 22 and the voltage control phase adjuster 23.
4 and the phase shifter 25.

Further, the signal U is supplied to the rectifier 26 and the integrator 2.
The control signal Uc of the variable gain amplifier 22 is formed via the signal line 7. The loop of the variable gain amplifier 22, the voltage control phase adjuster 23, the rectifier 26, the integrator 27, and the variable gain amplifier 22 has a function of making the magnitude of the output U constant regardless of the magnitude of the detected voltage.

The modulator 24 outputs a value obtained by multiplying the amplitude of the basic signal U by the modulation coefficient ε, and the phase shifter 2
Reference numeral 5 outputs a signal obtained by shifting the phase of the basic signal U by −θ (for example, 45 °). These two signals are added by the mixer 28 to obtain a signal Ue whose phase is shifted by an amount corresponding to the magnitude of the modulation coefficient ε as shown in FIG. I'm supposed to get Iexc. That is, the signal U is transferred to the phase shifter 25.
Through 45 ° and the coefficient ε ₁ or ε
Signal multiplied by the _{2 ε 1} U, the epsilon ₂ U (Fig. 8 epsilon _1>
The signals Ue ₁ and Ue _{2 obtained} by adding ε ₂ ) and the basic signal U have a delay angle θ ₁ > θ ₂ when ε ₁ > ε ₂ .

The modulation coefficient ε can be changed by adjusting the value of N of the control signal ω / N input to the modulator 24.

When the signal thus phase-shifted is obtained, the resonance frequency at a new phase is naturally obtained, and the first harmonic, that is, the vibration of i = 2 or the higher-order vibration is also obtained. Become.

When the relationship between the viscosity and the damping constant is actually measured by the above-mentioned apparatus by the 0th order vibration, as shown in FIG. 3 (a), and by the first order vibration, the result of FIG. 3 (b) is obtained. Becomes,
FIG. 5 shows the two graphs superimposed on each other.

Here, in order to correct the density error due to the viscosity of the density, if the viscosity-damping constant characteristic b 0 = f (η) at the 0th vibration is used, the viscosity η ₁ (specifically, Since the damping constant has a peak at about 100 mPas),
It has two viscosities for the same damping constant, and which value to use becomes a problem. If the first-order viscosity-damping constant characteristic b1 = g (η) is used, the viscosity can be uniquely obtained from the damping constant in a relatively wide range, but the peak still occurs at the portion of the viscosity η ₂ (about 700 mPas). Therefore, it faces the same problem as when the 0th order vibration is used.

As for a substance whose viscosity can be predicted to be about 700 mPas or less, the viscosity obtained from the first-order vibration can be used. The method of correcting the density error using the viscosity obtained from the vibration has a drawback that high accuracy cannot be expected.

The present invention has been proposed in view of the above-mentioned conventional circumstances. Even if the viscosity of vibration of a specific order has a peak in the damping constant characteristic, the viscosity of another specific order- It is an object of the present invention to provide a method for determining the viscosity in the vibration of the specific order by utilizing the damping constant characteristic.

[0016]

In order to achieve the above object, the present invention employs the following means. That is, if the viscosity-damping constant characteristic of the test substance at a vibration of a specific order has a peak point, is the damping constant of the test substance at a vibration of another specific order larger than the value corresponding to the peak point? The viscosity is determined by determining to which region of the peak point the damping constant obtained at the frequency of the specific order belongs, based on the smallness.

When the 0th order used directly for the density measurement is used as the above-mentioned specific order and the 1st order is used as the above-mentioned other specific order, the viscosity can be measured in the vibration mode used for the density measurement, and the accuracy is improved. Can be expected.

[0018]

1 is a diagram showing an outline of the present invention.

As shown in FIG. 5, when the U-shaped tube of the vibration type densitometer is driven by changing the viscosity of the test substance in the 0th mode, the viscosity has a peak at viscosity η = η ₁ The curve b0 = f (.eta.) Is obtained. When the viscosity of the test substance having the attenuation constant f (η ₀ ) is obtained using this curve,
The viscosity will have two values, η ₁₀ and η ₂₀ .

Therefore, the viscosity-damping constant curve b1 = g (η) is obtained by changing the viscosity by using the first-order vibration mode in addition to the viscosity-damping constant curve by the 0th-order vibration mode. The viscosity - the attenuation constant curve b1 = g (η), the attenuation constant which corresponds to the viscosity _{η = η 1 g (η 1} )
Becomes Therefore, the above viscosity-damping constant curve b0 = f
Two viscosities η obtained from the damping constant f (η ₀ ) of (η)
Which of ₁₀ and η ₂₀ is used can be determined by whether the damping constant g (η) in the first-order drive mode is larger or smaller than g (η ₁ ). That is, the damping constant g
When (η) is larger than g (η ₁ ), the viscosity η ₂₀ on the right side of the peak is obtained, and when the damping constant g (η) is smaller than g (η ₁ ), the viscosity η _{10 on} the left side of the peak is obtained. It is a power value.

In the above, when the viscosity is determined from the zero-order viscosity-damping constant curve b0 = f (η), the use of the first-order viscosity-damping constant curve b1 = g (η) is explained. However, in the same way, when determining the viscosity from the first-order viscosity-damping constant curve b1 = g (η), the zero-order viscosity-damping constant curve b0 = f (η) can be used.

That is, the first-order viscosity-damping constant curve b1
= G (η) peaks near the viscosity η ₂ and the viscosity η ₁
A curve showing two viscosities for the same damping constant from around the point where is exceeded. Therefore, the damping constant g (η ₀ )
Which of the two viscosities η ₃₀ and η ₄₀ obtained above is to be used is determined by the viscosity-damping constant curve b 0 =
Judge from f (η). That is, 0-order viscosity for the same analyte - attenuation constant curve b0 = f (η) from the resulting attenuation constant f (eta) is f (η ₂₎ left viscosity eta ₃₀ peaks when larger, but also , The damping constant f (η) is f
When it is smaller than (η ₂ ), the viscosity η ₄₀ on the right side of the peak is a value to be obtained.

For example, the U-shaped tube 10 is caused by the 0th order vibration.
Is oscillating at a frequency of about 200 to 350 Hz,
The above first-order vibration is 6.2673 times that of the above-mentioned zero-order vibration.
It vibrates at a frequency of about 1253 to 2194 Hz, and it is conceivable that the viscosity characteristic may change in these two frequency bands depending on the measurement target. On the other hand, the above-mentioned density measurement is currently performed by using the 0th-order vibration having a large amplitude. Since the density measurement is performed by the 0th-order vibration, the viscosity that causes a density error is also the 0th-order vibration. It can be expected that accurate correction can be achieved by using the viscosity below.

Table 1 is a table summarizing the experimental data from the above viewpoints. That is, 0 for each of the viscosity standard solution (Sample A) [Newtonian substance] and Samples B to E.
The damping constant was measured for the next vibration and the first vibration, and
While the viscosity was obtained using the graphs (a) and (b) (or a specific formula), the density of each sample was measured by zero-order vibration. The density obtained as described above is corrected with the viscosities obtained by the 0th-order vibration and the 1st-order vibration, respectively, by using the graph of FIG. 4 (or a specific equation) to obtain the corrected density.

Comparing the corrected density thus obtained with the true value of the density (obtained by the Wardung pycnometer method), in particular, for the substances of Samples B to E, it was obtained from the zero-order vibration. It can be understood that the corrected density obtained by using the viscosity is much higher in accuracy.

The Newtonian substance and a liquid whose shear rate (shear rate) are proportional to shear stress (shear stress) are liquids whose viscosity characteristics do not change depending on the frequency band. A non-Newtonian substance is a liquid whose shear rate (shear rate) is not proportional to shear stress (shear stress) like a polymer substance, and is also a liquid whose viscosity characteristics do not change depending on the frequency band.

[0027]

[Table 1]

As described above, according to the present invention, even when the viscosity-damping constant characteristic of a specific order has a peak, that is, when the viscosity has two viscosity values with respect to a certain damping constant. Also, the viscosity can be determined by using the viscosity-damping constant characteristics of other orders. Further, vibration of the same order can be used for the measurement of the density and the measurement of the viscosity, and the measurement accuracy can be improved.

[Brief description of drawings]

FIG. 1 is a viscosity-damping constant characteristic graph in each mode for explaining the principle of the present invention.

FIG. 2 is a conceptual diagram of a vibration type densitometer.

FIG. 3 is a graph showing a density error due to viscosity in zero-order vibration and first-order vibration.

FIG. 4 is a graph in which the two graphs of FIG. 3 are displayed together.

FIG. 5 is a viscosity-damping constant characteristic graph in each mode.

FIG. 6 is a diagram showing a model of a vibration mode of a U-shaped tube.

FIG. 7 is a conventional circuit diagram.

FIG. 8 is a waveform diagram showing a phase shift according to the related art.

─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-57-184950 (JP, A) JP-A-9-166465 (JP, A) JP-A-59-99332 (JP, A) JP-A-9- 222387 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 9/00 G01N 11/00

Claims (2)

(57) [Claims] 1. A method for determining a viscosity required when correcting a density error based on a viscosity of a test substance with respect to a measurement result of a vibration type densitometer, wherein the viscosity of the test substance in a vibration of a specific order- If the damping constant characteristic has a peak point, whether the damping constant measured using the viscosity-damping constant characteristic of the test substance at the vibration of another specific order is larger or smaller than the damping constant corresponding to the peak point. Based on, the viscosity determined from the damping constant obtained by the vibration of the specific order, the viscosity is determined by determining which region of the larger side or the smaller side the peak belongs to. Viscosity determination method. 2. The viscosity determining method according to claim 1, wherein the 0th order is used as the specific order, and the 1st order is used as the other specific order.